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Does the Foucault pendulum in Fairchild ever stop moving?

Dartmouth's Foucault pendulum
Photo by Joseph Mehling '69

Last updated 03/19/10

Yes, when the electricity goes off. The pendulum uses an electronic mechanism to keep it going; otherwise it would lose its energy due to friction and stop after a few hours.

When you stand near the pendulum, you hear clicks with each swing. This sound is produced by relays controlled by an electric eye – two pairs of lights and sensors that recognize when the beam of light is interrupted by the passing of the cable – that turn a magnet on and off to give a little pull. With this boost, the period of the pendulum’s swing is 9.3 seconds.

The pendulum stands just over 70 feet high, has a bob weighing 260 pounds, and takes 34.7 hours to complete its rotation around the compass on the floor. Supplied by the California Academy of Sciences, it was installed in Fairchild when the tower was completed in the 1974, overseen by the late physics professor Allen King. Professor King was a lover of historic scientific instruments and the driving force behind the Dartmouth Collection of Scientific Instruments, now curated by history Professor Richard Kremer. The description on display in front of the pendulum is the original one written by Professor King when the pendulum was put up.

The Foucault Pendulum is a classic physics experiment, which made it a natural choice to use as a display in Fairchild. In March 1851, French physicist Jean Bernard Leon Foucault famously set up a pendulum in the Paris Pantheon. That device had a bob with a pointer attached that traced patterns in sand, thus demonstrating the rotation of the earth. (For a visual demonstration, see this short time-lapse video of a pendulum at work).

Before Foucault, people knew the earth rotated but had no way to see it. Foucault’s innovation was recognizing that a pendulum swinging directly north and south would appear to an observer to move its oscillation plane as the Earth rotated beneath it.

Watch Dartmouth professor of physics and astronomy James LaBelle discuss the science and philosphy of Foucault’s invention while seated next the Dartmouth pendulum:

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Last updated: 03/19/10